Process for preparing an unsaturated carboxylic acid salt

ABSTRACT

Catalytic process for preparing an α,β-ethylenically unsaturated carboxylic acid salt, comprising reacting an alkene and carbon dioxide in the presence of a carboxylation catalyst and releasing the α,β-ethylenically unsaturated carboxylic acid salt with a base, the carboxylation catalyst being a transition metal complex, which comprises a structurally constrained bidentate P,X ligand, wherein X is selected from the group consisting of P, N, O, and carbene, the P and X atom are separated by 2 to 4 bridging atoms, and wherein the bridging atoms are part of at least one 5- to 7-membered cyclic substructure. A further catalytic processes for preparing α,βethylenically unsaturated carboxylic acid derivatives from CO 2  and an alkene is provided.

This application is a National Stage of PCT/EP2015/060535, which was filed on May 13, 2015. This application is based upon and claims the benefit of priority to European Application No. 14168612.1, which was filed on May 16, 2014.

The present invention relates to a catalytic process for preparing an α,β-ethylenically unsaturated carboxylic acid salt, comprising reacting an alkene and carbon dioxide in the presence of a carboxylation catalyst and releasing the α,β-ethylenically unsaturated carboxylic acid salt with a base. More particularly, the invention relates to a process for preparing sodium acrylate by direct carboxylation of ethene with carbon dioxide (CO₂). Acrylic acid and derivatives thereof are important industrial chemicals and monomer units for production of water-absorbing resins, called superabsorbents.

The direct addition of CO₂ onto ethylene to give acrylic acid is industrially unattractive due to thermodynamic limitations (ΔG=42.7 kJ/mol at 298 K) and the unfavorable equilibrium, which at room temperature is virtually completely to the side of the reactants (K₂₉₈=7×10⁻⁷). On the other hand, the formation of sodium acrylate and water from CO₂, ethylene and sodium hydroxide is thermodynamically favored (ΔG=−56.2 kJ/mol at 298 K, K₂₉₈=7.1×10⁹). By using a base, it is thus possible to convert the α,β-ethylenically unsaturated acid to the salt thereof and thus to shift the equilibrium to the side of the products. The reaction, however, is kinetically inhibited and therefore requires a homogeneous or heterogeneous carboxylation catalyst (Buntine et al., Organometallics 2007, 26, 6784).

The stoichiometric coupling of CO₂ and ethene at homogeneous Nickel complexes has been known since more than 30 years (Hoberg et al., J. Organomet. Chem. 1983, C51). The formation of nickelalactones as intermediates has been discussed, e.g. by Walther et al. (Chem. Commun. 2006, 23, 2510-2512). These do not spontaneously decompose by β-hydride elimination, as according to Walther's initial theory. Many nickelalactones are particularly stable and obtained in the form of solids by stoichiometric coupling of CO₂ and ethene (J. Organomet. Chem. 1983, C51; J. Organomet. Chem. 1982, 236, C28; Angew. Chem. Int. Ed. Engl. 1987, 26, 771). Some nickelalactones may even be isolated at room temperature in the form of stable solids (J. Organomet. Chem. 1982, 236, C28).

Nickelalactones are hydrolyzed by mineral acids in a non-catalytic reaction to yield a saturated carboxylic acid rather than an α,β-ethylenically unsaturated carboxylic acid. Concurrently, the nickel atom is oxidized.

Buntine et al. (Organometallics 2007, 26, 6784) and Walther et al. (Eur. J. Inorg. Chem. 2007, 2257) suggest that the initially postulated formation of acrylic acid by β-hydride elimination is energetically unfavored. This also explains for the stability of many nickelalactones. The β-hydride elimination postulated by Walther et al. and the equilibrium between nickelalactone and π-complex has never been realized experimentally.

WO 2011/107559 discloses a process for preparing an alkali metal or alkaline earth metal salt of an α,β-ethylenically unsaturated carboxylic acid, wherein a) an alkene, CO₂ and a carboxylation catalyst are converted to an alkene/CO₂/carboxylation catalyst adduct, b) the adduct is decomposed to release the carboxylation catalyst with an auxiliary base to give the auxiliary base salt of the α,β-ethylenically unsaturated carboxylic acid, c) the auxiliary base salt of the α,β-ethylenically unsaturated carboxylic acid is reacted to release the auxiliary base with an alkali metal or alkaline earth metal base to give the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid. The intermediate adduct is cleaved by means of an auxiliary base, for example of a tertiary amine, in order to prepare, in a first step, the ammonium salt of the α,β-ethylenically unsaturated carboxylic acid, which overcomes the fundamental thermodynamic limitation. In a second step, the ammonium cation is exchanged for sodium, for example by treatment with aqueous sodium hydroxide solution. The auxiliary base salt formed in the first step is separated from the reaction medium, e. g. by liquid-liquid phase separation.

Limbach et al. (WO 2013/098772, Chem. Eur. J. 2012, 18, 14017-14025) disclose a catalytic process for preparing an alkali metal or alkaline earth metal salt of an α,β-ethylenically unsaturated carboxylic acid, wherein a) a transition metal-alkene complex is reacted with CO₂ to give a metallalactone, b) the metallalactone is reacted with a base to give an adduct of the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid with the transition metal complex, the base being selected from alkali metal or alkaline earth metal hydroxides and alkali metal or alkaline earth metal superbases, and c) the adduct is reacted with an alkene to release the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid and regenerate the transition metal-alkene complex. In step c), the transition metal-alkene complex is regenerated and is available again for step a). This completes the catalytic cycle. Sodium acrylate was obtained at an overall yield of 1020% (2.55 mmol) based on the nickel (0.25 mmol). This yield was obtained in a process wherein the reaction conditions were varied in 18 cycles. Each cycle included increasing and decreasing CO₂ partial pressure, increasing and decreasing ethene partial pressure, and adding NaOtBu at decreased gas pressure.

It is an object of the present invention to provide further catalytic processes for preparing α,β-ethylenically unsaturated carboxylic acid derivatives from CO₂ and an alkene, in particular more efficient processes.

The invention provides a catalytic process for preparing an α,β-ethylenically unsaturated carboxylic acid salt, comprising

reacting an alkene and carbon dioxide in the presence of a carboxylation catalyst and releasing the α,β-ethylenically unsaturated carboxylic acid salt with a base, the carboxylation catalyst being a transition metal complex, which comprises a structurally constrained bidentate P,X ligand, wherein X is selected from the group consisting of P, N, O, and carbene, the P and X atom are separated by 2 to 4 bridging atoms, and wherein the bridging atoms are part of at least one 5- to 7-membered cyclic substructure.

The term “transition metal complex” used in the present application comprises, in a generic manner, all transition metal complexes through which the catalytic cycle is supposed to pass, i.e. transition metal-alkene complexes, metallalactones and adducts wherein the α,β-ethylenically unsaturated carboxylic acid salt coordinates to the transition metal.

In general, the transition metal complex comprises, as the active metal, at least one element of groups 4 (preferably Ti, Zr), 6 (preferably Cr, Mo, W), 7 (preferably Re), 8 (preferably Fe, Ru), 9 (preferably Co, Rh) and 10 (preferably Ni, Pd, Pt) of the Periodic Table of the Elements. Preference is given to nickel, cobalt, iron, rhodium, ruthenium, palladium, platinum, iridium, molybdenum, rhenium, tungsten. Particular preference is given to nickel, palladium, platinum, cobalt, iron, rhodium, ruthenium. Most preferably, the transition metal complex is a nickel or a palladium complex, in particular a nickel complex.

The role of the active metal consists in the activation of CO₂ and the alkene in order to form a C—C bond between CO₂ and the alkene. It is assumed that a metallalactone is formed within the catalytic cycle from the alkene, carbon dioxide and the transition metal complex. The expression “metallalactone” denotes, according to the exchange nomenclature (“a” nomenclature), a lactone (γ-lactone) in which a carbon atom has been exchanged for a metal atom. The expression “metallalactone” should be interpreted broadly and may comprise compounds with structures similar to the Hoberg complex mentioned at the outset, or related compounds of oligomeric or polymeric structure. The expression shall comprise isolable compounds and (unstable) intermediates.

The metallalactone can be illustrated by the following general formula

in which

-   M is the transition metal, -   L is a ligand, -   q is 1 or 2, and -   R^(a), R^(b) and R^(c) are each independently hydrogen, C₁₋₁₂-alkyl,     C₂₋₁₂-alkenyl, or R^(a) and R^(b) together with the carbon atoms to     which they are bonded are a mono- or diethylenically unsaturated, 5-     to 8-membered carbocycle.

It is assumed that the base deprotonates the metallalactone at the α-carbon atom.

In the process of the invention, a structurally constrained bidentate P,X ligand is used. X is selected from P, N, O, and carbene and the P and X atoms are separated by 2 to 4 bridging atoms. Structural constraint is imposed to the ligand by all of the bridging atoms being part of at least one 5- to 7-membered cyclic substructure. This includes embodiments where all bridging atoms, for example two bridging atoms, are part of a single cyclic substructure, or embodiments where one or more bridging atoms are part of a first cyclic substructure and one or more further bridging atoms are part of a second cyclic substructure.

The P and X atom being “separated by 2 to 4 bridging atoms” means that the shortest chain which connects the coordinating atoms P and X comprises 2 to 4 atoms. All of the 2 to 4 bridging atoms are preferably carbon atoms.

Preferably, the P,X ligand is a P,P ligand. The P,P ligand is preferably a bisphosphine wherein both coordinating phosphorous atoms are directly linked to three carbon atoms.

In preferred bidentate P,P ligands, each bridging atom directly linked to a P atom, together with the P atom to which it is linked, is part of a 5- to 7-membered cyclic substructure. Alternatively, two neighboring bridging atoms are part of a 5- to 7-membered cyclic substructure. The expression “two neighboring bridging atoms” indicates two bridging atoms directly linked by a bond.

The ligands described herein can occur as stereoisomers. They can be used, for example, as essentially pure stereoisomers or as their racemic mixtures. Stereoisomeric ligands differ from one another in the configuration at one or multiple stereogenic centers, i.e. at phosphorous or carbon atoms, and/or in the rotation about single bonds (atropisomerism). Any bidentate P,P ligand described herein is preferably a C₂-symmetric ligand.

Preferred bidentate P,P ligands are ligands of formula (IIa)

wherein

-   R⁶ is independently selected from CHR⁷ ₂, CR⁷ ₃, and     C₃-C₈-cycloalkyl, -   R⁷ is independently selected from linear C₁-C₄-alkyl, -   A¹ together with the carbon atoms to which it is bound and the     interjacent phosphorous atom forms a 5- to 7-membered cyclic     substructure, and -   R⁸ is independently selected from hydrogen, C₁-C₁₂-alkyl,     C₃-C₁₂-cycloalkyl, C₃-C₁₂-heterocycloalkyl, C₆-C₁₄-aryl,     C₆-C₁₄-heteroaryl, C₁-C₁₂-alkoxy, C₃-C₁₂-cycloalkoxy,     C₃-C₁₂-heterocycloalkoxy, C₆-C₁₄-aryloxy, and C₆-C₁₄-heteroaryloxy. -   A¹ is preferably selected from (CR⁸ ₂)_(m)— and (CR⁹═CR⁹)_(n)— with     both R⁹ being on the same side of the double bond, wherein R⁸ is     independently selected from H, C₁-C₃-alkyl, and —O—C₁-C₃-alkyl, R⁹     is selected from H and C₁-C₃-alkyl, or at least two R⁹ constitute a     bridge of one of the formulae:

-   m is 2 or 3, and n is 1 or 2. -   R⁶ is preferably CR⁷3. -   R⁷ is preferably methyl. -   R⁸ is preferably H. -   A¹ is preferably selected from ethylene, ethenylene, 1,2-phenylene,     1,2-naphthylene, 2,3-naphthylene, and the following formulae:

Preferred bidentate P,P ligands are ligands of formula (IIb)

wherein

-   R¹⁰ is independently selected from C₁-C₈-alkyl and C₃-C₈-cycloalkyl -   R¹¹ is independently selected from CHR¹⁰ ₂, CR¹⁰ ₃, and     C₃-C₈-cycloalkyl, -   X is independently selected from C—H, C—CH₃, and N, and -   A² together with the moieties X to which it is bound and the     interjacent carbon atoms forms a 5- to 7-membered cyclic     substructure. -   R¹⁰ is preferably independently selected from C₁-C₆-alkyl and     C₃-C₇-cycloalkyl and R¹¹ is CR¹⁰ ₃. -   R¹⁰ may, for example, be independently selected from linear     C₁-C₄-alkyl, in particular from linear C₁-C₂-alkyl. -   A² is preferably a —CH═CH— bridge. -   X is preferably CH.

Preferred bidentate P,P ligands are ligands of formula (IIc)

wherein

-   R¹³ and R¹⁴ are independently selected from C₃-C₈-cycloalkyl, e.g.,     C₅-C₇-cycloalkyl, and -   R¹⁵ is H, O—C₁-C₆-alkyl, or both R¹⁵ together constitute a CH═CH—     bridge. -   R¹⁵ is preferably H or OCH₃ and most preferably H.

In a particularly preferred process of the invention, the structurally constrained bidentate phosphorous ligand is selected from

The structurally constrained bidentate phosphorous ligand is, for example, selected from

-   Cy is cyclohexyl.

In addition to the above-described ligands, the transition metal complex may also have at least one further ligand selected from halides, amines, amides, oxides, phosphides, carboxylates, acetylacetonate, aryl- or alkylsulfonates, hydride, CO, olefins, dienes, cycloolefins, nitriles, aromatics and heteroaromatics, ethers, PF₃, phospholes, and mono-, di- and polydentate phosphinite, phosphonite, phosphoramidite and phosphite ligands.

In addition, at least one equivalent of the base or its conjugate acid may itself coordinate to the metal of the transition metal complex.

Any optional base and/or further ligand can be displaced when the alkene and carbon dioxide are reacted.

The transition metal complex may, for example, be obtained from the ligand and the transition metal or from the ligand and a transition metal source comprising the transition metal at oxidation state 0. Alternatively, the transition metal complex may, for example, be obtained by reducing a salt of the transition metal with a reducing agent, e.g., H₂, Mg, Na or Zn.

Useful transition metal sources and salts are commercially available and include, for example MX₂, MX₃, where X is selected from halide, pseudohalide, carboxylate, alkoxide, carbonate, sulfate, nitrate, hydroxide, acetylacetonate, cyclopentadiene, and the corresponding adducts with solvents such as ethers, DMSO, or water, and M is the active metal of the transition metal complex (e.g. [M(p-cymene)Cl₂]₂, [M(benzene)Cl₂]_(n), [M(COD)₂], [M(CDT)], [M(C₂H₄)₃], [MCl₂×H₂O], [MCl₃×H₂O], [M(acetylacetonate)₁₋₃], [M(DMSO)₄Cl₂]).

Palladium sources include, for example, PdL₂, PdL₄, LPdX₂, L₂PdX₂, L₂Pd₂X₂, LPd₂X₄, Pd₃X₆, L₃Pd₂, L₂Pd₂,

wherein

-   X is selected from halide, pseudohalide, carboxylate, alkoxide,     carbonate, sulfate, nitrate, hydroxide, acetylacetonate,     cyclopentadiene, alkyl, and aryl, and -   L is a neutral ligand selected from phosphine, amine, olefin,     carbonyl and nitrile,     and the corresponding adducts with solvents such as ethers, DMSO, or     water.

The palladium sources and salts are preferably selected from [Pd₂(Allyl)₂(Cl)₂], [Pd₂(Methallyl)₂(Cl)₂] [Pd(dba)₂], [Pd₂(dba)₃], PdCl₂, PdBr₂, PdI₂, Pd(NO₃)₂, PdSO₄ [Pd(OAc)₂], [Pd(PtBu₃)₂], [Pd(PCy₃)₂], [Pd(PoTolyl₃)₂], [Pd(PPh₃)₄], [Pd(COD)(Cl)(Me)], [Pd(Phen)(OAc)₂], [Pd₂(PtBu₃)₂(Br)₂], [Pd(C₆H₅CN)₂(Cl)₂], [Pd(PCy₃)₂(Cl)₂], [Pd(PPh₃)₂(Cl)₂], [Pd(norbornadiene)(Cl)₂], [Pd(TMEDA)(Cl)₂], [Pd(TMEDA)(CH₃)₂], [Pd₃(OAc)₆], [Pd(CF₃COO)₂], [Pd(Acetylactonate)₂] and [Pd(COD)(Cl)₂].

Nickel sources and salts include, for example, NiL₂, NiL₄, LNiX₂, L₂NiX₂, L₂Ni₂X₂ wherein X and L are as defined above and the corresponding adducts with solvents such as ethers, DMSO, or water.

The nickel sources and salts are preferably selected from [Ni(COD)₂], NiF₂, NiCl₂, NiBr₂, NiI₂, [Ni(OAc)₂], [Ni(Acetylactonate)₂], [Ni(Ph₃P)₂(Cl)₂], [Ni((PPh₂)₂Fc)(Cl)₂], [Ni₂(Methallyl)₂(Cl)₂], [Ni₂(allyl)₂(Cl)₂], [Ni(CO)₄], [Ni(PPh₃)₂(CO)₂], [Ni(NO₃)₂], [Ni(OH)₂], [Ni(PPh₃)₄], [Ni(CF₃COO)₂], [Ni(SO₄)], [Ni(2-ethylhexanoate)₂], [Ni(P(OPh)₃)₄], [Ni(C₇H₁₅COO)₂], [Ni(Cp)₂], [Ni(PCy₃)₂(Cl)₂], [Ni(PMe₃)₂(Cl)₂], [Ni(PBu₃)₂(Br)₂], and [Ni(dppe)(Cl)₂].

Suitable alkenes are those of the following general formula

wherein

-   R^(a), R^(b) and R^(c) are each independently hydrogen, C₁₋₁₂-alkyl,     C₂₋₁₂-alkenyl, or R^(a) and R^(b) together with the carbon atoms to     which they are bonded are a mono- or diethylenically unsaturated, 5-     to 8-membered carbocycle.

Suitable alkenes are, for example, ethene, propene, isobutene, butadiene, piperylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 2-butene, cyclopentene, cyclohexene, cycloheptene, cyclooctene, cyclooctadiene, or styrene. The alkene to be used in the carboxylation is generally gaseous or liquid under the reaction conditions.

In a preferred embodiment, the alkene is ethene. The process according to the invention makes it possible to obtain an acrylate.

In another embodiment, the alkene is piperylene and a sorbate is obtained.

The alkene partial pressure is for example between 0.5 and 200 bar, preferably between 1 and 100 bar, in particular between 2 and 80 bar. All pressures indicated herein are absolute pressures.

The CO₂ for use in the reaction can be used in gaseous, liquid or supercritical form. It is also possible to use carbon dioxide-comprising gas mixtures available on the industrial scale, provided that they are substantially free of carbon monoxide.

CO₂ and alkene may also comprise inert gases such as nitrogen or noble gases. Advantageously, however, the content thereof is below 10 mol %, based on the total amount of carbon dioxide and alkene in the reactor.

The carbon dioxide partial pressure is for example between 0.5 and 200 bar, preferably between 2 and 150 bar, in particular between 3 and 100 bar.

The molar ratio of carbon dioxide to alkene in the feed is generally 0.1 to 10 and preferably 0.5 to 5.

Preferably, the ratio of carbon dioxide partial pressure to alkene partial pressure is in the range from 0.1 to 10, for example, in the range from 0.5 to 5, in particular in the range from 1 to 4.

Any base that is capable of converting the metallalactone into an α,β-ethylenically unsaturated carboxylic acid salt that may be associated to the transition metal is suited as the base in the process according to the invention. In particular, any base that is capable of deprotonating the metallalactone under the conditions at which the alkene and carbon dioxide are reacted is suited as the base in the process according to the invention.

Although it is possible to isolate the metallalactone from the reaction mixture and to treat the isolated metallalactone with the base, it is generally preferred to carry out the reaction as a one-pot reaction. Thus, the base may be present in the reaction medium while the alkene and carbon dioxide are reacted, or the base may be added afterwards to the reaction medium. Certain bases, in particular nucleophilic bases, can react with carbon dioxide to form fairly stable adducts and the base is not available for the cleavage of the metallalactone. In these cases, the carbon dioxide pressure should be relieved before the base is added. Non-nucleophilic bases are largely unreactive towards carbon dioxide and can be included in the initial reaction medium or added to the reaction medium without prior carbon dioxide pressure relief.

Preferably, the base is an anion base. Most preferably, the anion base comprises a metal cation. The metal cation is preferably selected from alkali metals and alkaline earth metals.

The base is for example selected from hydrides, amides, alkoxides, and aryloxides, preferably from alkali metal or alkaline earth metal hydrides, amides, alkoxides, and aryloxides.

Suitable alkali metal or alkaline earth metal hydrides are, for example, lithium hydride, sodium hydride, potassium hydride, magnesium hydride, and calcium hydride.

Suitable alkali metal or alkaline earth metal amides are, for example, LiNMe₂, LiNEt₂, LiN(iPr)₂, NaNMe₂, NaNEt₂, NaN(iPr)₂, KNMe₂, KNEt₂, KN(iPr)₂, (Me=Methyl; Et=Ethyl; iPr=Isopropyl). The suitable amides also include silicon-containing amides such as sodium hexamethyldisilazide (NaHMDS), potassium hexamethyldisilazide (KHMDS) or lithium hexamethyldisilazide (LiHMDS).

Suitable alkali metal or alkaline earth metal alkoxides are derived from alcohols of the formula R¹⁰⁰OH. Suitable R¹⁰⁰ residues are branched or unbranched, acyclic or cyclic alkyl residue having 1-16 carbon atoms, preferably 1-12 carbon atoms, which are unsubstituted or wherein individual carbon atoms may each independently also be replaced by a heteroatom selected from the group of O and >N.

Exemplary R¹⁰⁰ residues are benzyl, methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, tert-butyl, 1-(2-methyl)propyl, 1-(2-methyl)butyl, 1-pentyl, 1-(2-methyl)pentyl, 1-hexyl, 1-(2-ethyl) hexyl, 1-heptyl, 1-(2-propyl)heptyl, 1-octyl, 1-nonyl, 1-decyl, 1-undecyl, 1-dodecyl, C₃-C₁₀-cycloalkyl which is unsubstituted or may bear a C₁-C₄-alkyl group, for example cyclopentyl, methylcyclopentyl, cyclohexyl, methylcyclohexyl, cycloheptyl, cyclooctyl.

Preferred R¹⁰⁰ residues are secondary or tertiary residues. The terms “secondary” or “tertiary” refer to the branching of the carbon atom bound to the hydroxyl group of R¹⁰⁰OH. Exemplary secondary or tertiary R¹⁰⁰ residues are 2-propyl, 2-butyl, tert-butyl, cyclopentyl, methylcyclopentyl, cyclohexyl, methylcyclohexyl, cycloheptyl, cyclooctyl.

Particularly preferred R¹⁰⁰ residues are tertiary residues, i.e. tert-butyl.

Preferable alkali metal or alkaline earth metal alkoxides are sodium methoxide, sodium ethoxide, sodium isopropoxide, sodium tert-butoxide, and sodium isobutoxide. In the case the base is an alkoxide, the alcohol from which the alkoxide is obtainable by deprotonation, may serve as the solvent.

Sodium tert-butoxide is a preferred base.

Suitable aryloxides are, for example phenolates and naphtholates. The aryloxide may, for example, be an aryloxide that corresponds to the general formula (I)

wherein

-   R is selected from F, Cl, Br, I, C₁-C₁₆-alkyl, C₃-C₁₆-cycloalkyl,     and C₁-C₁₆-fluoroalkyl, and two vicinal R may constitute a     C₃-C₅-hydrocarbylene bridge that is optionally substituted by one to     four substituents which are independently selected from F, Cl, Br,     I, C₁-C₁₆-alkyl, C₃-C₁₆-cycloalkyl, and C₁-C₁₆-fluoroalkyl, and -   r is an integer selected from 1 to 5,     wherein at most two R are F.

For example, the C₃-C₅-hydrocarbylene bridge is an unsaturated C₄-hydrocarbylene bridge, preferably with two conjugated double bonds. When the C₄-hydrocarbylene bridge has two conjugated double bonds, the aryloxide of formula (I) is a naphthyloxide. The C₄-hydrocarbylene bridge is optionally substituted by one to four substituents which are independently selected from F, Cl, Br, I, C₁-C₁₆-alkyl, C₃-C₁₆-cycloalkyl, and C₁-C₁₆-fluoroalkyl.

In a first aspect of the process according to the invention, the aryloxide may, for example, correspond to one of the following general formulae (Ia-1), (Ia-2), and (Ia-3)

wherein

-   x is 0, 1, or 2, -   R² is methyl, and -   R³ is independently C₁-C₁₆-alkyl or C₃-C₁₆-cycloalkyl.

Among these, the aryloxides that correspond to the formulae (Ia-2) and (Ia-3) are preferred. x is preferably 1 or 2 in formulae (Ia-2) and (Ia-3).

F is preferably ortho or meta to the oxygen (O⁻ substituent) in the aryloxides that correspond to formula (Ia-2). Aryloxide that corresponds to formula (Ia-2) with F being ortho to O⁻, and x being 1 or 2, preferably 1, is particularly preferred.

In the first aspect, the aryloxide is preferably an alkali metal, alkaline earth metal or zinc aryloxide. Preferred alkali metal cations of the alkali metal aryloxide are Na⁺, Li⁺, and K⁺. Preferred alkaline earth metal cations of the alkaline earth metal aryloxide are Mg²⁺ and Ca²⁺. The aryloxide is most preferably a sodium aryloxide.

In the first aspect, the aryloxide may, for example, be selected from sodium 2-fluorophenolate, sodium 3-fluorophenolate, sodium 4-fluorophenolate, sodium 2,6-difluorophenolate, sodium 2,4-difluorophenolate, sodium 2-chlorophenolate, sodium 3-chlorophenolate, sodium 4-chlorophenolate, sodium 2-methylphenolate, sodium 2,6-dimethylphenolate, and sodium 1-naphtholate. Sodium 2-fluorophenolate, sodium 3-fluorophenolate, sodium 2-chlorophenolate, and sodium 3-chlorophenolate are particularly preferred.

In a preferred second aspect of the process according to the invention, the aryloxide is a phenyloxide wherein the hydrogen atoms at position 2 and position 6 of the phenyl residue are both substituted by a C₁-C₁₆-alkyl or C₃-C₁₆-cycloalkyl residue. In the second aspect, the aryloxide thus preferably corresponds to the general formula (I)

wherein

-   R is independently selected from C₁-C₁₆-alkyl, and     C₃-C₁₆-cycloalkyl, -   r is an integer selected from 2 to 5,     and one R is at position 2 and another R is at position 6 of the     phenyl ring of general formula (I). This means that both R are at     the positions that are ortho to O⁻.

In the second aspect, R may be any primary, secondary or tertiary alkyl residue. R is preferably independently selected from C₁-C₆-alkyl, for example, from methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, tert-butyl, most preferably from methyl, 2-propyl, and tert-butyl.

r is preferably 2, 3, or 4, in particular 2 or 3.

Particularly preferred aryloxides of the second aspect of the process according to the invention are selected from 2,6-di-C₁-C₄-alkyl phenoxides, and 2,6-di-C₁-C₄-alkyl-4-C₁-C₄-alkyl phenoxides.

In preferred 2,6-di-C₁-C₄-alkyl phenoxides, the C₁-C₄-alkyl residues are preferably independently selected from methyl, 2-propyl, and tert-butyl.

In preferred 2,6-di-C₁-C₄-alkyl-4-C₁-C₄-alkyl phenoxides, the number of carbon atoms comprised by any of the alkyl residues at positions 2 and 6 of the phenyl ring is the same as or higher than the number of carbon atoms comprised by the alkyl residue at position 4 of the phenyl ring.

In the second aspect of the process according to the invention, the aryloxide is preferably an alkali metal, alkaline earth metal or zinc aryloxide, most preferably a sodium aryloxide.

Specific examples of aryloxides of the second aspect according to the invention are sodium 2,6-dimethyl phenoxide, sodium 2,6-diisopropyl phenoxide, sodium 2-methyl-6-tert-butyl phenoxide, sodium 2,6-di-tert-butyl phenoxide, sodium 2,6-dimethyl phenoxide, sodium 2,6-dimethyl-4-tert-butyl phenoxide, sodium 2,4,6-trimethyl phenoxide, sodium 2,6-di-tert-butyl-4-methyl phenoxide, sodium 2,6-di-tert-butyl-4-sec-butyl phenoxide, and 2,4,6-tri-tert-butyl phenoxide.

The aryloxide can be added in solid form or as a solution.

The aryloxide is consumed stoichiometrically when the alkene and carbon dioxide are reacted to obtain the α,β-ethylenically unsaturated carboxylic acid salt. The aryloxide is protonated such that its conjugate acid, an arylhydroxide, is obtained as a byproduct. Quantitative consumption of the aryloxide can be prevented by reacting the arylhydroxide with an alkaline material which is capable of deprotonating the arylhydroxide such that the aryloxide is recycled. Accordingly, the process of the invention preferably comprises regenerating the aryloxide by adding an alkaline material.

The amount of aryloxide used in the process according to the invention is generally 5 to 95% by weight, preferably 20 to 60% by weight, and most preferably 5 to 15% by weight, based on the overall reaction medium in the reactor.

It is possible to use the aryloxide in substoichiometric amounts based on the carboxylation catalyst. Even when substoichiometric amounts of aryloxide are used, it is possible to obtain α,β-ethylenically unsaturated carboxylic acid salt in excess as based on the catalyst concentration, if the aryloxide is regenerated by addition of the alkaline material.

If an alkaline material is used that is capable of regenerating the aryloxide under the conditions at which the alkene and carbon dioxide are reacted, the alkaline material may be present in the reaction medium while the alkene and carbon dioxide are reacted to the α,β-ethylenically unsaturated carboxylic acid salt. Such alkaline material is for example added into the carboxylation reactor while the alkene and carbon dioxide are reacted.

If the alkaline material is added outside of the carboxylation reactor, i.e. at low carbon dioxide partial pressure, alkaline materials that are inactivated at the conditions of the reaction between alkene and carbon dioxide, i.e. at high carbon dioxide partial pressure, may be used. These alkaline materials include alkali metal or alkaline earth metal hydroxides, oxides and alkoxides.

The alkaline material is, for example, selected from elemental alkali metal, alkali metal, alkaline earth metal, or zinc hydroxides, carbonates, hydrogencarbonates, oxides, phosphides, silanolates, alkyls, and aryls.

Suitable elemental alkali metals are sodium, lithium, and potassium. The preferred elemental alkali metal is sodium.

Suitable alkali metal and alkaline earth metal hydroxides are, for example, sodium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide.

Suitable carbonates are, for example, lithium carbonate, sodium carbonate, potassium carbonate, calcium carbonate and zinc carbonate.

Suitable alkali metal hydrogencarbonates are, for example, sodium hydrogencarbonate or potassium hydrogencarbonate.

Suitable alkali metal and alkaline earth metal oxides are, for example, lithium oxide, sodium oxide, calcium oxide and magnesium oxide. Preference is given to sodium hydroxide.

Suitable alkali metal or alkaline earth metal phosphides are, for example, those of the formula M²PR¹⁰¹ ₂ in which M² is an alkali metal or an equivalent of an alkaline earth metal, and R¹⁰¹ is C₁₋₁₂-alkyl or C₆₋₁₀-aryl, for example KPPh₂ or NaPPh₂ (Ph=Phenyl).

Suitable alkali metal or alkaline earth metal silanolates are, for example, those of the formula M²OSi(C₁₋₄-Alkyl)₃ in which M² is an alkali metal or an equivalent of an alkaline earth metal, for example NaOSiMe₃.

Suitable alkali metal alkyls or aryls are, for example, lithium alkyl and lithium aryl compounds, such as methyllithium, n-butyllithium, sec-butyllithium, tert-butyllithium, phenyllithium, where the benzene ring may bear substituents at any position (e.g. OCH₃, CH₂NMe₂, CONR₂), cyclohexyllithium, where the cyclohexyl ring may comprise heteroatoms (e.g. O, N, S), ethyllithium, lithium pentadienyl, lithium 2-furanyl, lithium 2-thiophenyl, lithium ethynyl. Also suitable are sodium alkyl and sodium aryl compounds, such as sodium cyclopentadienyl.

The suitable alkaline earth metal alkyls and aryls include magnesium alkyl and magnesium aryl compounds (Grignard reagents) of the general formula R¹⁰²MgX, where R¹⁰² may be one of the alkyl and aryl residues listed above for the lithium alkyl and lithium aryl compounds and X may be F, Cl, Br, or I.

The suitable zinc alkyls and aryls include zinc alkyl and zinc aryl compounds of the general formula R¹⁰³ZnX, where R¹⁰³ may be one of the alkyl and aryl residues listed above for the lithium alkyl and lithium aryl compounds and X may be F, Cl, Br, or I.

Suitable alkaline materials are also alkali metals, in particular sodium. The deprotonation of the arylhydroxide is then coupled with a redox reaction. The alkali metal is oxidized to the alkali metal cation and the oxygen-bound proton of the arylhydroxide is reduced to hydrogen.

Some of the aforementioned bases, i.e. alkali metal or alkaline earth metal hydrides, amides, and alkoxides, are also capable of regenerating the aryloxide by deprotonating the arylhydroxide. Suitable alkaline material thus also include the aforementioned alkali metal or alkaline earth metal hydride, amide, and alkoxide bases.

A particularly preferred alkaline material is sodium hydroxide.

It may happen that part of the carboxylation catalyst is deactivated by oxidation of the active metal. The deactivation reduces the overall efficiency of the process. Preferably a reducing agent is added. Apparently, the reducing agent reactivates the deactivated carboxylation catalyst by reduction of the oxidized active metal. Thus, the alkene and carbon dioxide are preferably reacted in the presence of a reducing agent. Any reducing agent which is capable of reducing the deactivated carboxylation catalyst is suitable as the reducing agent. Preferable reducing agents are H₂, Mg, Na and Zn.

In preferred embodiments, the transition metal complex, and preferably also any intermediate reaction product formed during the reaction of the alkene and carbon dioxide is present in homogeneous solution in the reaction medium in the form of complex-type compounds.

The reaction medium wherein the alkene and carbon dioxide are reacted preferably comprises 0.1 to 20000 ppm by weight, preferably 1 to 1000 ppm by weight, in particular 5 to 500 ppm by weight of transition metal, based on the total weight of the reaction medium.

Preferably, the reaction medium comprises an aprotic organic solvent. Suitable aprotic organic solvents are in principle those which (i) are chemically inert with regard to the carboxylation of the alkene, (ii) in which the base and the carboxylation catalyst have good solubility, and (iii) which are immiscible or only have limited miscibility with the polar solvent as defined below. Useful aprotic organic solvents are therefore in principle chemically inert, nonpolar solvents, for instance aliphatic, aromatic or araliphatic hydrocarbons, or ethers, for example octane and higher alkanes, benzene, toluene, xylene, chlorobenzene, and anisole. The reaction medium may, for example, comprise an aprotic organic solvent selected from aromatic hydrocarbons, halogenated aromatic hydrocarbons, alkylated aromatic hydrocarbons, alkanes, ethers, dimethylformamide, dimethyl sulfoxide and mixtures thereof.

Examples of suitable ethers are dimethylether, diethylether, di-tert-butylether, di-n-butylether, tetrahydrofuran and 2-methyl-tetrahydrofuran.

Particularly preferred aprotic organic solvents are selected from

-   -   cyclic alkyl ethers with 4 to 8 carbon atoms,     -   dialkyl ethers with 2 to 12 carbon atoms,     -   cycloalkyl alkyl ethers with 4 to 12 carbon atoms,     -   aryl alkyl ethers with 7 to 16 carbon atoms,     -   biaryls with 12 to 16 carbon atoms,     -   diaryl oxides with 12 to 16 carbon atoms,     -   C₁-C₈-alkyl esters of C₆-C₁₀-aryl monocarboxylic acids,     -   di-C₁-C₈-alkyl esters of C₆-C₁₀-aryl dicarboxylic acids,     -   dialkyl carbonates with 3 to 13 carbon atoms,     -   diethers consisting of an dioxyalkylene residue with 2 to 8         carbon atoms and two C₁-C₈-alkyl residues,     -   benzenes wherein 1 to 4 hydrogen atoms are substituted by 1 to 4         C₁-C₄-alkyl residues,     -   halogenated benzenes,     -   alkanes with 5 to 18 carbon atoms,     -   and their mixtures.

The most preferred aprotic organic solvents are selected from tetrahydrofuran, anisole, phenyl butyl ether, dibutyl ether, mixtures of biphenyl and diphenylether, cyclopentyl methyl ether, dibutyl phthalate, butyl benzoate, diethyl carbonate, dibutyl glycol ether, toluene, 2-methyl tetrahydrofuran, monochlorobenzene, and their mixtures.

Albeit the reaction of the alkene and carbon dioxide in the presence of the carboxylation catalyst and releasing the α,β-ethylenically unsaturated carboxylic acid salt with the base is preferably carried out in an aprotic polar solvent, the reaction is tolerant of minor concentrations of polar solvents, in particular, protic polar solvents such as specified below, for example, water. The reaction medium, may thus, for example, contain traces of water or be saturated or partially saturated with water. The term “saturated” refers to the maximum solubility of water in the reaction medium without forming a distinct aqueous phase, at a temperature of 25° C. and pressure of 1 bar. For example, the reaction is tolerant of traces of water up to the water saturation concentration in certain ethers, in particular aryl alkyl ethers with 7 to 16 carbon atoms, e.g., anisole. The water tolerance of the process according to the invention is important, as lower grade and therefore less expensive aprotic organic solvents can be used.

The reactors used may in principle be all reactors which are suitable in principle for gas/liquid reactions or liquid/liquid reactions at the given temperature and the given pressure. Suitable standard reactors for liquid-liquid reaction systems are specified, for example, in K. D. Henkel, “Reactor Types and Their Industrial Application”, in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley VCH Verlag GmbH & Co KGaA, DOI: 10.1002/14356007.b04_087, chapter 3.3 “Reactors for gas-liquid reactions”. Examples include stirred tank reactors, tubular reactors or bubble columns.

The process may be performed as a discontinuous process or as a discontinuous process. In the discontinuous process, the ligand, the transition metal which may, for example, be in the form of the transition metal source, the base, carbon dioxide and the alkene are given into the reactor. Preferably, gaseous carbon dioxide and gaseous alkene are passed into the reactor at the desired pressure. After the reaction has slowed down, the pressure may be reduced.

The process may, for example, be performed at pressures between 1 and 300 bar, preferably between 1 and 200 bar, in particular between 1 and 150 bar. The process may, for example, be performed at temperatures between −20 and 300° C., preferably between 20 and 250° C., in particular between 40 and 200° C.

In order to achieve good mixing of the reactants and of the medium comprising the carboxylation catalyst and the base, suitable apparatuses can be used. Such apparatuses may be mechanical stirrer apparatuses with one or more stirrers, with or without baffles, packed or nonpacked bubble columns, packed or nonpacked flow tubes with or without static mixers, or other useful apparatuses known to those skilled in the art for these process steps. The use of baffles and delay structures is explicitly included in the process according to the invention.

CO₂, alkene and the base can be fed to the reaction medium either together or spatially separately. Such a spatial separation can be accomplished, for example in a stirred tank, in a simple manner by means of two or more separate inlets. When more than one tank is used, for example, there may be different media charges in different tanks. Separation of the addition of the CO₂ and alkene reactants in terms of time is also possible in the process according to the invention. Such a time separation can be accomplished, for example, in a stirred tank by staggering the charging with the reactants. In the case of use of flow tubes or apparatus of a similar kind, such charging can be effected, for example, at different sites in the flow tube; such a variation of the addition sites is an elegant way of adding the reactants as a function of residence time. In the process of the present invention, there is no need of separately feeding the CO₂, the alkene and the base to the reaction medium.

One or more immiscible or only partly miscible liquid phases can be used. The optional use of supercritical media and ionic liquids and the establishment of conditions which promote formation of such states are explicitly incorporated into the process. The optional application of phase transfer catalysis and/or the use of surfactants are explicitly incorporated into the process according to the invention.

In a preferred embodiment, the α,β-ethylenically unsaturated carboxylic acid salt is removed from the reaction medium. The removal of the salt preferably comprises a liquid-liquid phase separation into a first liquid phase in which the α,β-ethylenically unsaturated carboxylic acid salt is enriched, and a second liquid phase in which the carboxylation catalyst and the base which may also be in the form of its conjugate acid, are enriched. The first and second liquid phases are obtained by bringing the reaction medium which preferably comprises the aprotic organic solvent into contact with a polar solvent, for example, with the polar solvent as specified below, preferably with an aqueous solution, in particular with water.

“Enriched” is understood to mean a partition coefficient P of each of the carboxylation catalyst, the base and its conjugate acid of >1.

$P = \frac{\begin{matrix} \left\lbrack {{Concentration}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{carboxylation}} \right. \\ {{catalyst}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{second}\mspace{14mu}{liquid}\mspace{14mu}{phase}} \end{matrix}\mspace{31mu}}{\begin{matrix} \left\lbrack {{Concentration}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{carboxylation}} \right. \\ \left. {{catalyst}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{first}\mspace{14mu}{liquid}\mspace{14mu}{phase}} \right\rbrack \end{matrix}}$ $P = \frac{\left\lbrack {{Concentration}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{base}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{second}\mspace{14mu}{liquid}\mspace{14mu}{phase}} \right.}{\left\lbrack {{Concentration}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{base}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{first}\mspace{14mu}{liquid}\mspace{14mu}{phase}} \right\rbrack}$ $P = \frac{\begin{matrix} \left\lbrack {{Concentration}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{conjugate}\mspace{14mu}{acid}\mspace{14mu}{of}} \right. \\ {{the}\mspace{14mu}{base}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{second}\mspace{14mu}{liquid}\mspace{14mu}{phase}} \end{matrix}}{\begin{matrix} \left\lbrack {{Concentration}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{conjugate}\mspace{14mu}{acid}\mspace{14mu}{of}} \right. \\ \left. {{the}\mspace{14mu}{base}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{first}\mspace{14mu}{liquid}\mspace{14mu}{phase}} \right\rbrack \end{matrix}}$

Each of these partition coefficients is preferably 0 and more preferably 20, for example

The second liquid phase preferably comprises an aprotic organic solvent as specified above, in particular an aprotic organic solvent in which the solubility of the base and its conjugate acid is high. When the base is an aryloxide, the aprotic organic solvent is thus preferably selected from the aprotic organic solvents specified above, e.g., from aromatic aprotic organic solvents, as for example from

-   -   aryl alkyl ethers with 7 to 16 carbon atoms,     -   biaryls with 12 to 16 carbon atoms,     -   diaryl oxides with 12 to 16 carbon atoms,     -   C₁-C₈-alkyl esters of C₆-C₁₀-aryl monocarboxylic acids,     -   di-C₁-C₈-alkyl esters of C₆-C₁₀-aryl dicarboxylic acids,     -   benzenes wherein 1 to 4 hydrogen atoms are substituted by 1 to 4         C₁-C₄-alkyl residues,     -   halogenated benzenes,     -   and their mixtures;

When the base is an aryloxide, the aprotic organic solvent comprised by the second liquid phase is most preferably selected from anisole, phenyl butyl ether, dibutyl ether, mixtures of biphenyl and diphenylether, dibutyl phthalate, butyl benzoate, toluene, monochlorobenzene, and their mixtures.

The base is generally selected by a simple experiment in which the partition coefficient of the base and of its conjugate acid is determined experimentally under the planned process conditions.

The liquid-liquid phase separation is preferably promoted by the additional use of a polar solvent in which the α,β-ethylenically unsaturated carboxylic acid salt has good solubility and which has zero or only limited miscibility with the second liquid phase in which the carboxylation catalyst and the base are enriched. The polar solvent should be selected such that the polar solvent is present in enriched form in the first liquid phase. “Enriched” is understood to mean a proportion by weight of >50% of the polar solvent in the first liquid phase based on the total amount of polar solvent in both liquid phases. The proportion by weight is preferably >90%, more preferably >95% and most preferably >97%. The polar solvent is generally selected by simple tests in which the partition of the polar solvent in the two liquid phases is determined experimentally under the process conditions.

It is thus preferable to bring the reaction medium, e.g., the crude reaction product, into contact with the polar solvent. The crude reaction product is formed in the reaction underlying the process according to the invention. The crude reaction product usually comprises the carboxylation catalyst, the α,β-ethylenically unsaturated carboxylic acid salt, the conjugate acid of the base formed as a byproduct, e.g., the aryl hydroxide that is formed as a byproduct when the base is an aryloxide, and usually some unreacted base, alkene, carbon dioxide, and the optional aprotic organic solvent.

Preferred substance classes which are suitable as polar solvents are water, alcohols, diols and the carboxylic esters thereof, polyols and the carboxylic esters thereof, sulfones, sulfoxides, open-chain or cyclic amides, and mixtures of the substance classes mentioned.

Examples of suitable alcohols are methanol, ethanol, 1-propanol, isopropanol, tert-butanol and butanol. Examples of suitable diols and polyols are ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, 1,3-propanediol, 2-methyl-1,3-propanediol, 1,4-butanediol, dipropylene glycol, 1,5-pentanediol, 1,6-hexanediol and glycerol.

Examples of suitable sulfoxides are dialkyl sulfoxides, preferably C₁- to C₆-dialkyl sulfoxides, especially dimethyl sulfoxide.

Examples of suitable open-chain or cyclic amides are formamide, N-methylformamide, N,N-dimethylformamide, N-methylpyrrolidone, acetamide and N-methylcaprolactam.

In a preferred embodiment, the polar solvent is water or an alcohol. In a particularly preferred embodiment, the polar solvent is water, methanol, isopropanol or tert-butanol, most preferably water.

Suitable selection of the base and optionally of the polar solvent and/or of the aprotic organic solvent which is immiscible or has only limited miscibility therewith, for example, achieves the effect that the carboxylation catalyst, the base and its conjugate acid are enriched in the second liquid phase. The second liquid phase comprising the carboxylation catalyst, the base and its conjugate acid can be separated by phase separation from the first liquid phase comprising the α,β-ethylenically unsaturated carboxylic acid salt and be recycled to the reactor without further workup or after regenerating the base, i.e. the aryloxide, by adding the alkaline material. Owing to the rapid separation of the α,β-ethylenically unsaturated carboxylic acid salt from the catalyst, its decomposition to carbon dioxide and alkene is suppressed. In addition, losses of active metal, base and its conjugate acid are minimized, as the catalyst, base and its conjugate acid are retained in the second liquid phase.

If desired, traces of water comprised by the second liquid phase can be removed by contacting the second liquid phase with a drying agent. The second liquid phase may, for example, be passed through a column that is filled with the drying agent.

As mentioned above, the reaction underlying the process according to the invention, i.e. the reaction of the alkene and carbon dioxide in the presence of the carboxylation catalyst and the release of the α,β-ethylenically unsaturated carboxylic acid salt with the base, is tolerant of minor concentrations of water.

By appropriate selection of an aprotic organic solvent having incomplete miscibility with water the amount of water remaining physically dissolved in the organic phase after phase separation can be kept low. It is thus possible to recycle the second liquid phase into the carboxylation reactor without removing water dissolved in the second liquid phase. For example, the solubility of water in the aprotic organic solvent at a temperature of 25° C. and a pressure of 1 bar is less than 5%, preferably less than 2%, in particular less than 1% by weight.

To remove the first liquid phase, the procedure may be to only conduct the first liquid phase out of the carboxylation reactor and to leave the second liquid phase within the carboxylation reactor. Alternatively, a liquid-liquid mixed-phase stream can be conducted out of the carboxylation reactor and the liquid-liquid phase separation can be performed in a suitable apparatus outside the carboxylation reactor. The two liquid phases are generally separated by gravimetric phase separation. Suitable examples for this purpose are standard apparatus and standard methods which can be found, for example, in E. Müller et al., “Liquid-Liquid Extraction”, in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH Verlag GmbH & Co. KGaA, DOI:10.1002/14356007.b03_06, chapter 3 “Apparatus”. In general, the first liquid phase enriched with the α,β-ethylenically unsaturated carboxylic acid salt is heavier and forms the lower phase. The second liquid phase can subsequently be recycled into the carboxylation reactor.

In embodiments, wherein the aryloxide is regenerated by adding alkaline material, the alkaline material can, for example, be added to the reaction medium, to the second liquid phase or to any mixture of the first and the second liquid phase. The regeneration of the base (aryloxide) is preferably performed in the liquid or supercritical phase at pressures between 1 and 150 bar, preferably at pressures between 1 and 100 bar, more preferably at pressures between 1 and 60 bar. The temperature may, for example, be between −20 and 300° C., preferably between 20 and 250° C., more preferably between 40 and 200° C. The reaction conditions at which the base (aryloxide) is regenerated may be the same as or different than those at which the alkene and carbon dioxide are reacted.

If the regeneration of the aryloxide is conducted under the conditions at which the alkene and carbon dioxide are reacted, i.e. in the carboxylation reactor, an alkaline material that is not inactivated under the conditions of the reaction, i.e. an alkali metal or alkaline earth metal hydride, is used.

Regeneration of the aryloxide at a reduced carbon dioxide partial pressure, e.g. a carbon dioxide partial pressure of 0 to 500 mbar, preferably 0 to 100 mbar, i.e. outside of the carboxylation reactor, allows a nucleophilic base such as an alkali metal or alkaline earth metal hydroxide, in particular sodium hydroxide, to be used as the alkaline material. The reaction of the arylhyroxide with the alkali metal or alkaline earth metal hydroxide regenerates the alkali metal or alkaline earth metal aryloxide; an equivalent of water is produced as a by-product.is. The regeneration of the aryloxide may be driven to completion by removing, i.e. evaporating, water.

The separation of the α,β-ethylenically unsaturated carboxylic acid salt is preferably achieved by phase separation. The alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid can, for example, be separated in a polar aqueous phase from an organic phase. The optional use of supercritical media and ionic liquids, and the establishment of conditions which promote the formation of such states, is explicitly incorporated into the process. Optional changes in temperature and pressure that facilitate the separation of the phases are explicitly included by the process of the invention.

The regeneration of the aryloxide and the phase separation may be carried out together in one step. An aqueous phase comprising the alkaline material, i.e. an aqueous solution of the alkaline material, is preferably added to the reaction medium after the alkene and carbon dioxide have been reacted, such that two phases are formed and the arylhydroxide is reconverted to the aryloxide. Liquid-liquid extraction can be effected in all apparatus suitable for this purpose, such as stirred vessels, extractors or percolators. An aqueous phase is obtained, which comprises an aqueous solution of the alkali metal or alkaline earth metal salt of the α,β-ethylenically unsaturated carboxylic acid, and an organic phase which comprises the aryloxide and the carboxylation catalyst.

One specific process according to the invention is a continuous process comprising

-   -   a first step that is carried out in a reaction zone, e.g., in a         carboxylation reactor, wherein the reaction of the alkene, e.g.,         ethene, and carbon dioxide in the presence of the carboxylation         catalyst and the release of the α,β-ethylenically unsaturated         carboxylic acid salt with the base is carried out in the         reaction medium comprising the aprotic organic solvent,         -   the second liquid phase being recycled from the second step,             the alkene, and the carbon dioxide are fed into the reaction             zone, and         -   the crude reaction product is discharged from the reaction             zone; and     -   a second step that is carried out in a phase separation zone,         e.g., in a liquid-liquid phase separator, wherein         -   the crude reaction product obtained from the first step is             fed into the phase separation zone,         -   the first liquid phase in which the α,β-ethylenically             unsaturated carboxylic acid salt is enriched is discharged             from the phase separation zone, and         -   the second liquid phase in which the carboxylation catalyst,             the unconverted aryloxide and the arylhydroxide are             enriched, is discharged from the phase separation zone and             recycled into the first step with removing some or all of             the polar solvent, e.g., water, comprised by the second             liquid phase or without removing any of the polar solvent;             and             wherein

-   the crude reaction product is mixed with the polar solvent before it     is fed into the phase separation zone and/or when it is fed into the     phase separation zone, in order to obtain the first liquid phase and     the second liquid phase, and

-   the alkaline material, e.g., sodium hydroxide, is added to the crude     reaction product that is discharged from the reaction zone and/or     into the phase separation zone and/or into the second liquid phase     that is discharged from the phase separation zone.

The invention is illustrated in detail by the examples which follow.

In the examples, the following abbreviations are used:

-   DMF dimethylformamide -   Ex. Example -   NaHMDS sodium bis(trimethylsilyl)amide -   NaOtBu sodium tert-butoxide -   NaOMe sodium methoxide -   Ni(COD)₂ bis(cyclooctadiene)nickel(0) -   PhCl monochlorobenzene -   THF tetrahydrofuran -   TON turnover number with respect to transition metal -   dtbpe 1,2-bis(di-tert-butylphosphino)ethane -   dtbpb 1,4-bis(di-tert-butylphosphino)butane -   dcppe 1,2-bis(dicyclopentylphosphino)ethane -   dcppp 1,3-bis(dicyclopentylphosphino)propane -   dcppb 1,4-bis(dicyclopentylphosphino)butane -   dcpe 1,2-bis(dicyclohexylphosphino)ethane -   dcpp 1,3-bis(dicyclohexylphosphino)propane -   dcpb 1,4-bis(dicyclohexylphosphino)butane -   diprpe 1,2-bis(diisopropylphosphino)ethane -   diprpp 1,3-bis(diisopropylphosphino)propane -   diprpb 1,4-bis(diisopropylphosphino)butane -   Ferrocene-1     1-diphenylphosphino-2-diphenylphosphino-4-tert-butyl-cyclopentadienyl-1′-diisopropylphosphino-3′-tert-butyl-cyclopentadienyl     iron -   Ferrocene-2     1-diphenylphosphino-2-diphenylphosphino-4-tert-butyl-cyclopentadienyl-1′-diphenylphosphino-3′-tert-butyl-cyclopentadienyl     iron -   Ferrocene-3 1,1′-bis(diisopropylphosphinocyclopentadienyl) iron -   Ferrocene-4 1,1′-bis(dicyclohexylphosphinocyclopentadienyl) iron -   iPr-MeOBIPHEP     2,2′-bis(diisopropylphosphino)-6,6′-dimethoxy-1,1′-biphenyl -   Ph-BPE 1,2-bis-((2R,5R)-diphenylphospholano)ethane -   Triphos 1,1,1-tris(diphenylphosphinomethyl)ethane

General Procedure

In a glovebox, 30 mL of solvent, transition metal source, ligand, base, and optionally alkaline material, as indicated in or below Tables 1 to 8, were transferred under an argon atmosphere into a charging cartridge sealable by valves at both ends. When Zinc (finely powdered, <10 μm, Sigma Aldrich Cat. No. 209988) was added, it was directly transferred into the autoclave.

The autoclave was charged outside of the glovebox with ethene. One end of the charging cartridge was connected to an inlet port of the autoclave. The other end of the charging cartridge was connected to a nitrogen line. Both valves were opened such that the content of the charging cartridge was transferred into the autoclave. The autoclave was pressurized at room temperature with ethene, and then with carbon dioxide. The equilibration time was 15 minutes with each gas (the partial pressures of CO₂ (p(CO₂)) and ethene (p(C₂H₄)) are indicated below Tables 1 to 8. The contents were stirred at 500 rpm in the autoclave for the time and at the temperature given in or below Tables 1 to 8. The autoclave was cooled to room temperature and opened. The reaction medium was transferred into a glass bottle having a volume of 100 mL. The autoclave was washed with D₂O (15 mL) and internal standard (NMe₄I, 25.1 mg, 0.125 mmol, or 2,2,3,3-d₄-3-(trimethylsilyl)propionic acid, 28.7 mg, 0.167 mmol, both in 5 mL D₂O) was added. The autoclave was washed with D₂O (5 mL). All D₂O used for washing the autoclave was combined with the reaction medium. Diethyl ether (40 mL) was added to the combined phases and 2 mL of the aqueous phase were centrifuged in order to improve phase separation. The amount of sodium acrylate was determined by ¹H-NMR spectroscopy (200 MHz, 70 scans) and the TON determined from the amount of sodium acrylate.

TABLE 1 Ligand Base Example (0.22 mmol) (20 mmol) TON 1 DuanPhos NaH 10 2 DuanPhos NaHMDS 2 3a DuanPhos NaOtBu 14 3b DuanPhos NaOMe 2 Reaction conditions: THF, 0.2 mmol Ni(COD)₂, p(CO₂): 20 bar, p(C₂H₄): 10 bar, 100° C., 20 h.

The results of Table 1 demonstrate that the process according to the invention can be carried out with different bases, i.e. hydride, amide, and alkoxide.

TABLE 2 Ferrocene- iPr- T DuanPhos BenzP* dtbpe QuinoxP* 1 MeOBIPHEP [° C.] Ex. TON Ex. TON Ex. TON Ex. TON Ex. TON Ex. TON 80 4 21 7 10 10¹⁾ 6 13 11 16¹⁾ 1 19 2 100 5 24 8 35 11¹⁾ 12 14 14 17¹⁾ 3 20 4 120 6 21 9 31 12¹⁾ 13 15 7 18¹⁾ 7 21 3 Reaction conditions: 10 mmol sodium 2-fluoro phenolate, 0.22 mmol ligand, THF, 0.2 mmol Ni(COD)₂, p(CO₂): 10 bar, p(C₂H₄): 5 bar, 20 h ¹⁾reference example

The results shown in Table 2 demonstrate that the ligands used in the process according to the invention are more efficient than dtbpe.

TABLE 3 DuanPhos TangPhos Binapine dtbpe BenzP* solvent Ex. TON Ex. TON Ex. TON Ex. TON Ex. TON Methanol 22 0 29 0 36 0 43¹⁾ 0 50 0 DMF 23 5 30 6 37 1 44¹⁾ 2 51 2 PhCl 24 5 31 12 38 9 45¹⁾ 13 52 9 Heptane 25 6 32 10 39 11 46¹⁾ 3 53 7 Dioxane 26 10 33 11 40 7 47¹⁾ 6 54 13 Toluene 27 15 34 15 41 12 48¹⁾ 6 55 17 THF 28 16 35 14 42 10 49¹⁾ 9 56 8 Average 8.1 9.7 7.1 5.6 8 TON Reaction conditions: 10 mmol sodium 3-fluoro phenolate, 0.22 mmol ligand, 0.2 mmol Ni(COD)₂, p(CO₂): 10 bar, p(C₂H₄): 5 bar, 80° C., 20 h ¹⁾reference example

The results shown in Table 2 and 3 demonstrate that the ligands used in the process according to the invention are more efficient than dtbpe.

TABLE 4 Ex. Ligand TON 57¹⁾ dtbpe 9.2 58¹⁾ dcpe 3.9 59 iPr-DUPHOS 1.3 60 QuinoxP* 4.4 61 iPr-MeOBIPHEP 2.9 62¹⁾ Ferrocene-2 0.2 63 2,2′-Bis(dicyclohexyl- 11.4 phosphino)-1,1′-biphenyl 64¹⁾ Triphos 0.3 65 DuanPhos 15.8 66 Methyl-DUPHOS 1.8 67¹⁾ Ph-BPE 2 68 TangPhos 13.7 69¹⁾ dcppe × 2 HBF₄ 4.8 70¹⁾ dcppp × 2 HBF₄ 7.5 71¹⁾ 1-di-tert-butylphosphino-2- 0.4 aminoethane × 2 HBF₄ 72¹⁾ dtbpb × 2 HBF₄ 0.2 73¹⁾ dcppb × 2 HBF₄ 1.5 74¹⁾ dcpb × 2 HBF₄ 1.3 75¹⁾ dcpp × 2 HBF₄ 4.7 76¹⁾ Ferrocene-3 1.1 77¹⁾ Ferrocene-1 2 78 BenzP* 8.7 79¹⁾ Bis(diphenylphosphino- 0.7 methyl)phenylphosphine 80¹⁾ 1-di-tert-butylphosphino-2- 0.6 aminoethane 81¹⁾ diprpp 5.7 82¹⁾ diprpe 6 83¹⁾ diprpb 1.8 84¹⁾ Ferrocene-4 1.3 85 Binapine 10.4 Reaction conditions: 10 mmol sodium 3-fluoro phenolate, 0.22 mmol ligand, THF, 0.2 mmol Ni(COD)₂, p(CO₂): 10 bar, p(C₂H₄): 5 bar, 80° C., 20 h ¹⁾reference example

TABLE 5 Reducing Bases agent Ex. Ligand (10 mmol) (10 mmol) (10 mmol) TON 86 DuanPhos sodium 2-fluorophenolate — — 28 87 DuanPhos sodium 2-fluorophenolate NaH — 23 88 DuanPhos sodium 2-fluorophenolate — Zn 54 89 DuanPhos sodium 2-fluorophenolate NaH Zn 43 90 BenzP* sodium 2-fluorophenolate — — 65 91 BenzP* sodium 2-fluorophenolate NaH — 73 92 BenzP* sodium 2-fluorophenolate — Zn 77 93 BenzP* sodium 2-fluorophenolate NaH Zn 85 Reaction conditions: 0.11 mmol ligand, THF, 0.1 mmol Ni(COD)₂, p(CO₂): 10 bar, p(C₂H₄): 5 bar, 100° C., 20 h

TABLE 6 Zn Ex. ligand (mmol) base solvent T (° C.) TON 94 BenzP* 10 sodium 2- THF 120 107 fluorophenolate (30 mmol) 95 BenzP* 10 sodium 2,6- THF 80 1 difluorophenolate (30 mmol) 96 BenzP* 10 sodium 2- THF 80 4 methylphenolate (30 mmol) 97 BenzP* 10 sodium 2,6- THF 80 4 dimethylphenolate (30 mmol) 98 BenzP* 10 sodium 2,6- THF 80 43 dimethylphenolate (20 mmol) Reaction conditions: 0.22 mmol ligand, 0.2 mmol Ni(COD)₂, p(CO₂): 20 bar, p(C₂H₄): 10 bar, 20 h

The results of Tables 1 to 6 show that an efficient catalytic preparation of α,β-ethylenically unsaturated carboxylic acid derivatives from CO₂ and an alkene is achieved in the process of the invention.

TABLE 7 Ex ligand TON Pd leaching¹⁾ [ppm] 99

7 2 Reaction conditions: 0.011 mmol ligand, 10 mmol sodium 2,6-di-tert-butyl-4-methyl phenolate, 0.01 mmol Pd(PPh₃)₄, 30 mL anisole, p(CO₂): 20 bar, p(C₂H₄): 10 bar, 145 °C., 20 h. ¹⁾The reaction medium was treated as described in the general procedure. The amount of palladium leaching into the aqueous phase was determined by ICP-MS.

The result of Table 7 shows that the amount of transition metal leaching from crude reaction products into aqueous phases is very small.

EXAMPLES 100 TO 103

The amount of base and conjugate acid being transferred from crude reaction products into aqueous phases was tested. The amount of aprotic organic solvent (THF), base and conjugate acid comprised by the crude reaction products are shown in Table 8.

TABLE 8 amount of conjugate amount of acid of the Ex. base THF base base 100 101

30 mL 30 mL 18.6 mmol <0.1 mmol   1.4 mmol >9.9 mmol 102 103

30 mL 30 mL 19.4 mmol  4.6 mmol 0.6 mmol 5.4 mmol

Each crude reaction product was transferred from the autoclave into a glass bottle having a volume of 100 mL. The autoclave was washed with D₂O (15 mL) and internal standard (NMe₄I, 25.1 mg, 0.125 mmol, or 2,2,3,3-d₄₋₃-(trimethylsilyl)propionic acid, 28.7 mg, 0.167 mmol, both in 5 mL D₂O) was added. The autoclave was washed with D₂O (5 mL). All D₂O used for washing the autoclave was combined with the reaction medium. Diethyl ether (40 mL) was added to the combined phases and 2 mL of the aqueous phase were centrifuged in order to improve phase separation. The water phase was filtered and then evaporated. The solid residue was dissolved in D₂O and ¹H NMR spectra were recorded of the D₂O solution thus obtained in order to determine the amount of base (aryloxide) and its conjugate acid (arylhydroxide) transferred into the aqueous phase. While large peaks were observed for the alkene protons of the acrylate, there were no or only very small peaks originating from aliphatic protons (i.e. from the methyl protons of the tert-butyl groups comprised by the aryloxide). This shows, that no or only very little aryloxide and arylhydroxide was transferred into the aqueous phase. 

The invention claimed is:
 1. A catalytic process for preparing an α,β-ethylenically unsaturated carboxylic acid salt, the process comprising reacting an alkene and carbon dioxide in the presence of a carboxylation catalyst and releasing the α,β-ethylenically unsaturated carboxylic acid salt with a base, wherein: the carboxylation catalyst is a transition metal complex comprising a structurally constrained bidentate P,P ligand; the P atoms of the P,P ligand are separated by 2 to 4 bridging atoms; the bridging atoms are contained in at least one 5- to 7-membered cyclic substructures; the base is at least one selected from the group consisting of a hydride, an amide, an alkoxide and an aryloxide; the alkene has the following formula:

and R^(a), R^(b) and R^(c) are each independently hydrogen, C₁₋₁₂-alkyl, C₂₋₁₂-alkenyl, or R^(a) and R^(b) together with the carbon atoms to which they are bonded are a mono- or diethylenically unsaturated, 5- to 8-membered carbocycle.
 2. The catalytic process according to claim 1, wherein: each bridging atom of the P,P ligand that is directly linked to a P atom, together with the P atom to which it is linked is contained in a 5- to 7-membered cyclic substructure; or two neighboring bridging atoms of the P,P ligand are contained in a 5- to 7-membered cyclic substructure.
 3. The catalytic process according to claim 1, wherein the structurally constrained bidentate P,P ligand is selected from the group consisting of: a ligand of formula (IIa):

wherein R⁶ is independently selected from the group consisting of CHR⁷ ₂, CR⁷ ₃, and a C₃-C₈-cycloalkyl, R⁷ is independently a linear C₁-C₄-alkyl, A¹ together with carbon atoms to which it is bound and an interjacent phosphorous atom forms a 5- to 7-membered cyclic substructure, and R⁸ is independently selected from the group consisting of hydrogen, a C₁-C₁₂-alkyl, a C₃-C₁₂-cycloalkyl, a C₃-C₁₂-heterocycloalkyl, a C₆-C₁₄-aryl, a C₆-C₁₄-heteroaryl, a C₁-C₁₂-alkoxy, a C₃-C₁₂-cycloalkoxy, a C₃-C₁₂-heterocycloalkoxy, a C₆-C₁₄-aryloxy, and a C₆-C₁₄-heteroaryloxy; a ligand of formula (IIb):

wherein R¹⁰ is independently selected from the group consisting of a C₁-C₈-alkyl and a C₃-C₈-cycloalkyl, R¹¹ is independently selected from the group consisting of CHR¹⁰ ₂, CR¹⁰ ₃, and a C₃-C₈-cycloalkyl, X is independently selected from the group consisting of C—H, C—CH₃, and N, and A² together with moieties X to which it is bound and interjacent carbon atoms forms a 5- to 7-membered cyclic substructure; and a ligand of formula (IIc):

wherein R¹³ and R¹⁴ are independently a C₃-C₈-cycloalkyl, and R¹⁵ is H, O—C₁-C₆-alkyl, or both R¹⁵ together constitute a —CH═CH— bridge.
 4. The catalytic process according to claim 3, wherein: the structurally constrained bidentate P,P ligand is a ligand of formula (IIa); and R⁶ is CR⁷ ₃.
 5. The catalytic process according to claim 3, wherein: the structurally constrained bidentate P,P ligand is a ligand of formula (IIa); and A¹ is selected from the group consisting of ethylene, ethenylene, 1,2-phenylene, 1,2-naphthylene, 2,3-naphthylene, and formula:


6. The catalytic process according to claim 3, wherein: the structurally constrained bidentate P,P ligand is a ligand of formula (IIb); and each R¹⁰ is independently selected from the group consisting of a C₁-C₆-alkyl and a C₃-C₇-cycloalkyl; and R¹¹ is CR¹⁰ ₃.
 7. The catalytic process according to claim 3, wherein: the structurally constrained bidentate P,P ligand is a ligand of formula (IIb); and A² is a —CH═CH— bridge.
 8. The catalytic process according to claim 3, wherein: the structurally constrained bidentate P,P ligand is a ligand of formula (IIc); and R¹⁵ is H or OCH₃.
 9. The catalytic process according to claim 3, wherein: the structurally constrained bidentate P,P ligand is a ligand of formula (IIb); and X is CH.
 10. The catalytic process according to claim 1, wherein the structurally constrained bidentate P,P ligand is selected from the group consisting of

wherein Cy represents cyclohexyl.
 11. The catalytic process according to claim 1, wherein the transition metal complex is a nickel complex.
 12. The catalytic process according to claim 1, wherein: the alkene is ethene; and the α,β-ethylenically unsaturated carboxylic acid is acrylic acid.
 13. The catalytic process according to claim 1, wherein the alkene and the carbon dioxide are reacted in the presence of a reducing agent.
 14. The catalytic process according to claim 1, wherein the reacting occurs in the presence of a reaction medium comprising an aprotic organic solvent.
 15. The catalytic process according to claim 14, wherein the aprotic organic solvent is selected from the group consisting of a cyclic alkyl ether having 4 to 8 carbon atoms, a dialkyl ether having 2 to 12 carbon atoms, a cycloalkyl alkyl ether having 4 to 12 carbon atoms, an aryl alkyl ether having 7 to 16 carbon atoms, a biaryl having 12 to 16 carbon atoms, a diaryl oxide having 12 to 16 carbon atoms, a C₁-C₈-alkyl ester of a C₆-C₁₀-aryl monocarboxylic acid, a di-C₁-C₈-alkyl ester of a C₆-C₁₀-aryl dicarboxylic acid, a dialkyl carbonate having 3 to 13 carbon atoms, a diether s consisting of a dioxyalkylene residue having 2 to 8 carbon atoms and two C₁-C₈-alkyl residues, a benzenes wherein 1 to 4 hydrogen atoms are substituted by 1 to 4 C₁-C₄-alkyl residues, a halogenated benzene, an alkane having 5 to 18 carbon atoms, and mixtures thereof.
 16. The catalytic process according to claim 1, wherein the base is an aryloxide.
 17. The catalytic process according to claim 16, wherein the aryloxide is selected from the group consisting of sodium 2-fluorophenolate, sodium 3-fluorophenolate, sodium 2-chlorophenolate, and sodium 3-chlorophenolate.
 18. The catalytic process according to claim 16, wherein the aryloxide is selected from the group consisting of sodium 2,6-dimethyl phenolate, sodium 2,6-diisopropyl phenolate, sodium 2-methyl-6-tert-butyl phenolate, sodium 2,6-di-tert-butyl phenoxide, sodium 2,6-dimethyl phenolate, sodium 2,6-dimethyl-4-tert-butyl phenolate, sodium 2,4,6-trimethyl phenolate, sodium 2,6-di-tert-butyl-4-methyl phenolate, sodium 2,6-di-tert-butyl-4-sec-butyl phenolate, and 2,4,6-tri-tert-butyl phenolate.
 19. The catalytic process according to claim 1, wherein: the reacting occurs in the presence of a reaction medium; the α,β-ethylenically unsaturated carboxylic acid salt is removed from the reaction medium; the removal of the α,β-ethylenically unsaturated carboxylic acid salt from the reaction medium comprises a liquid-liquid phase separation into a first liquid phase in which the α,β-ethylenically unsaturated carboxylic acid salt is enriched, and a second liquid phase in which the carboxylation catalyst, the base and a conjugate acid of the base are enriched; and, the first and second liquid phases are obtained by contacting the reaction medium with a polar solvent.
 20. The catalytic process according to claim 16, further, comprising regenerating the aryloxide by adding an alkaline material. 